The Labour party has made a "serious complaint" to the BBC about a lack of political balance in its news coverage as it attempts to reinvigorate Ed Miliband's leadership and counter what it sees as widespread media bias in favour of the David Cameron-led coalition.

The Observer has learned that Labour chiefs have written to the corporation raising concerns that its party spokesmen are not receiving their fair share of airtime at a time when they are neck and neck with the Tories in opinion polls.

It is understood that party officials have monitored invitations, and time given, to senior Tory and Liberal Democrat figures on the BBC's main news outlets against that allotted to Labour counterparts. Their analysis has shown that Labour has been represented less than half as often as the coalition.

While acknowledging that the Tories and Lib Dems are in government and should therefore take precedence, they believe the imbalance has left Labour struggling to get its messages across. They have made their point to top BBC executives in forceful terms.

One source familiar with the dispute said Labour had made a "very serious complaint" to the relevant authories.

Labour insiders insisted on Saturday that the party was not "declaring war" on the BBC or "whingeing", but merely holding the corporation to its obligation to show impartiality.

However, they stressed there was concern in the Miliband camp that the BBC was becoming an "echo chamber" for a press that is dominated by newspapers "determined for David Cameron to succeed".

Miliband's team are also growing increasingly frustrated at the way newspapers, particularly in the pro-coalition Rupert Murdoch-owned stable, run prominent stories about their own opinion polls only when the Tories enjoy a surge but not when Labour is comfortably ahead, as it was for most of last year.

The BBC refused to be drawn into the dispute but strongly denied bias. A BBC News spokesperson said: "Our news coverage is impartial and features a range of voices."

Labour is no stranger to confrontations with the BBC, which often finds itself harangued by political parties alleging bias. In June 2003, Tony Blair's director of communications, Alastair Campbell, demanded explanations of the corporation's coverage of the Iraq weapons dispute. In the same month he stormed on to Channel 4 News to take on presenter Jon Snow over Iraq.

The latest dispute comes to light as Miliband and his team prepare a series of new year interventions on policy after a miserable few weeks in which the Tories have regained a lead in several polls.

A poll of polls in the Independent on Saturday showed the Tories ahead of Labour for the first time in 14 months. It put the Conservatives on 39%, (up four points), with Labour on 38% (down one). The Lib Dems were unchanged on 12%.

Many Labour MPs are frustrated and worry that the party is lagging well behind the coalition on key indicators of economic credibility. Behind the scenes, pressure is growing for Miliband to be bolder, with many urging him to back a transaction tax on banks – a move resisted by the shadow chancellor, Ed Balls. Senior figures are concerned that unless Miliband can stem the flow of negative coverage and push ahead of the Tories, there is a danger he could sink to a low point from which it would be difficult to recover.

Strategists say Labour is determined in the new year to show a "sharpened focus" on the "cost of living crisis" facing families on low to middle earnings – those who are in the so-called "squeezed middle".

In a first move the party calls on the government to ban train operating companies from raising rail fares above strict limits. Ahead of new year prices rises that will see many fares soar by as much as 9%, Labour says its plan would limit increases to a maximum of 1% above inflation.

Maria Eagle MP, Labour's shadow transport secretary, said: "Instead of price rises being limited to 1% above inflation, some of the most popular commuter routes will see the cost of a season ticket soar by hundreds of pounds."

Further initiatives to highlight Labour's push for fairness are expected in coming weeks as the party announces plans to rein in bank credit card charges and deliver a fairer deal on university fees, housing and tax credit reform.

In a new year message on the NHS, the shadow health secretary, Andy Burnham, is promising to keep up the fight to kill off the government's health reforms, now passing through the Lords.